1. Field of the Invention
This invention is designed to simulate moisture sorption processing in wildland fuels to provide moisture content values for fire danger and fire behavior predictions. This invention is keyed to fuel characteristics of moisture response timelag, surface area to volume ratio, shape factor, size, radiation properties, and water sorption characteristics with the additional advantage of not being subject to non-reproducibility which advantage eliminates calibration problems inherent in natural fuels.
2. Review of prior art
Numerous attempts have been made to simulate moisture contents of forest fuels through use of analog devices. Basswood slats have been used to estimate the moisture content of fine fuels; arrays of 1/2-inch diameter ponderosa pine dowels have been used to represent intermediate fuels; and arrays of 2-inch diameter ponderosa pine dowels have been used to represent heavy fuels. However, use of wood to indicate forest moisture has several deficiencies:
1. the response characteristics of wood are highly variable; they may vary as much as a factor of 10 between samples and as much as a factor of 2 between samples taken from the same piece of wood. PA1 2. Exposure and aging produce discoloration, checking and splitting, and mass loss. Discoloration changes the radiation characteristics, and thus with exposure and aging a given set of environmental conditions will result in different fuel moistures on different dates. Checking and splitting, particularly on the ends, exposes more surface area for a given volume of fuel and also produces a shift in response characteristics. Loss of mass results in an absolute shift in calibration over a period of time.
Thus, the use of wood in a fuel moisture analog causes inaccuracies because of non-reproducible response characteristics between units, a change in response characteristics over time, and an absolute shift in calibration over time. These problems are not adequately countered by use of wood moisture meters. Thus, an object of this invention is to produce a sensor with reproducible response characteristics. A further object is to produce a sensor the response of which does not deteriorate with use and the passage of time. Achievement of these two objectives would permit a single calibration to be used for every unit rather than having to guess at the calibration of each unit as must be done presently.
The moisture content of the organic forest floor has been indicated by Duff hygrometers, but since these use wood as the basic sensor, they are subject to the same errors produced by response characteristics and variability in volumetric changes due to moisture content as the wood analogs noted above. Although the instrument could be calibrated more readily than could methods involving direct use of wood, it was not able to represent forest fuels other than the forest floor.
Computational analogs based on regression analysis of wood response to the environment, linear models, and direct use of the diffusion theory have been developed to represent forest fuel moisture, but these analogs are generally subject to errors introduced by the inability of calculations to include nonlinear interactive processes and by non-representative data, either by reduction of data to fuel level or by climatologically limited data.
Recently, a fuel moisture analog with an electrical readout was developed which implicitly represented several fuel sizes ranging from small fuels to large fuels, but this device has two major deficiencies: (1) the values of moisture content are not specifically keyed to forest fuel's response characteristic, and (2) the materials employed are inert.
It was an object of this invention to overcome the deficiencies of the previous art in the following respects: (1) the sensor is keyed directly to dead forest fuels and fire-danger rating; (2) the specified response characteristics can be maintained from unit to unit; (3) the materials are active and respond similarly to wood; (4) the geometric configuration of the analog can be prescribed so that its form is the same as the fuel it is representing; and (5) the analog has an electrical readout of the intra-pore humidity which represents the moisture content of the fuel.